We will chronically implant microelectrode arrays in monkey somatosensory hand cortex to study neuron responses, with the focus on effects of applying two stimuli that vary in spatial and temporal proximity. Results will reveal contextual effects that are relevant to human tactile processing and may relate to the basis of object discrimination. The long-term objective of the study is to determine how and where the somatosensory system mediates object perception and related abilities in order to better characterize the aspects of somatosensory processing that underlie neurological phenomena like phantom limb syndrome or recovery from stroke or other neurological damage to the somatosensory system. The idea guiding the proposal is that object discrimination requires integration of information from across the hand, and this integration is likely to involve primary somatosensory cortex (area 3b). We hypothesize that responses of area 3b neurons will be modulated when two stimuli are nearer in spatial and temporal proximity, but that modulations will occur even when stimuli are separated by substantial distances. We base this hypothesis on 1) several lines of evidence that near stimuli (on adjacent digits or whiskers) applied closely in time modulate the neuron response to the stimulus in the receptive field, 2) interhemispheric interactions that can be unmasked in area 3b, and 3) preliminary studies indicating that such sources of surround modulation are not limited to spatially proximate stimuli. In Aim 1, we will mechanically stimulate paired finger locations to assess how modulation of neural responses may be related to spatial and temporal proximity of the two stimuli. In Aim 2, we will administer local lidocaine injections to block input from one hand and characterize responses to stimulation on the opposite hand, in order to deduce effects of normal levels of spontaneous activity from the opposite hemisphere on shaping 3b neuron activity. Thus, we will examine interactions between responses to stimuli delivered to sites across the hand or hands to investigate how neurons in somatosensory cortex respond to a stimulus in the receptive field, in the context of a distant stimulus. Relevance: The normal functions of neurons in the somatosensory system are relevant not only for understanding the normal sense of touch, but also to understanding recoveries from stroke or damage to the somatosensory cortex (i.e., understanding the extent to which processing is altered by lesions and to what extent normal function returns to the area with or without therapy). Future experiments (dorsal column lesions) are planned to test this further, but the proposed study is part of the foundational work necessary to interpret such future studies.